ARM: dts: imx6: phycore: Rely on PMIC reboot/reset handler

Due to the missing signal connection between i.MX6 WDOG_B pin and the
PMICs external reset, the internal i.MX6 watchdog is not able to reset
the phyCORE i.MX6 SoM properly. Thus disable the internal i.MX6 watchdog
to prevent unexpected PMIC settings after reset.

NOTE: This patch should not be backported as it might break existing
uses and fixes in bootloaders.

Signed-off-by: Andrej Picej <andrej.picej@norik.com>
Reviewed-by: Stefan Riedmüller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>